Polypropylene has widely been used in various fields including parts in automobile, parts for machines, parts for electric appliances, household commodities, kitchen utensils, packaging films and so on, wherein improvements in the performances thereof have been proposed by incorporation of additives therein which provide facilitation of requisite properties. For example, addition of an inorganic filler, such as talc, has been practised for improving stiffness and heat resistance. However, a plastic resin containing talc exhibits disadvantages, such as higher density, tendency to easy occurrence of surface damages and scratches on articles molded therefrom and tendency to whitening of the damaged portion.
In Japanese Patent Kokai Hei-6-93034 A (corresponding European Patent Application No. EP 573862 A2), a polypropylene product is described in which the processibility in molten state is improved. This polypropylene product exhibits, however, insufficient improvements in the mechanical properties, such as inferior flexural modulus and lower thermal deformation temperature.
In Japanese Patent Kokai Hei-4-202507 A, a process for producing a polypropylene resin in multiple polymerization staged in the presence of a catalyst having a specific solid titanium catalyst component is described, in which it is said that the resulting polypropylene resin is superior in the stiffness, heat resistance and so on. However, this process requires a prepolymerization of, such as, 3-methyl-1-butene, and is complicated. The polypropylene resin obtained by this process exhibits insufficient improvement in the stiffness.
In Japanese Patent Kokai Sho-59-172507 A, a process for producing a polypropylene resin superior in the stiffness, processibility and heat resistance is described, in which propylene is polymerized in two polymerization stages using a Ziegler catalyst. In this polypropylene resin, however, the improvements in the flexural modulus and in the stiffness are insufficient.
Japanese Patent Kokai Hei-4-370103 discloses a process for producing a polypropylene resin superior in the processibility and stiffness in a multi-stage polymerization using a specific catalyst. This polypropylene resin provides, however, insufficient improvement in the flexural modulus.
The object of the present invention is to provide a polypropylene resin composition, which is light-weighing and has very high values of stiffness and heat resistance, superior scratch-proof or anti-damaging property and better flowability upon molding or shaping with superior processibility, as well as injection molded articles made of such polypropylene resin composition.